The hypogonadism and infertility of the adult hypogonadal (hpg) mouse is correctable by intracerebral implantation of perinatal normal brain tissue that contains gonadotropin-releasing hormone (GnRH) neurons. This phenomonon depends upon the capacity of these implanted neurons to send axons to the median eminence and on their integration into appropriate host neural circuits. We have undertaken several in vivo studies to address these issues and now propose to combine these with in vitro methods in a concerted program to elucidate the factors underlying both pathfinding of GnRH axons and steroid regulation of their connectivity. We will first carry out a detailed analysis of GnRH axonal development from early embryonic to late postnatal stages. In parallel, we shall employ an insert chamber culture system to determine whether the mediobasal hypothalamus (MBH) provides chemotaxic or chemotropic signal(s) for GnRH axons. For continued in vivo analysis in the adult hpg we shall use purified olfactory epithelial-derived (OE) cells as a source of embryonic GnRH neuroblasts. Prior data from our laboratory suggest that it is likely that these cells can differentiate into 'adult' GnRH neurons and send axons to the median eminence. We shall now test whether the site of implantation of the OE cells is critical to their overall differentiation, axonal targeting, and capacity to support reproductive function. As a second focus we shall pursue our observation that estrogen (E2) has a neuromodulatory action on GnRH secretion in hpg male mice with grafts (hpg/POA). Studies will evaluate aromatizable vs. nonaromatizable steroids in sensitizing hpg/POA males to respond to neuromodulatory challenges such as nMDA. We will determine whether E2 treatment affects NMDARI receptor expression in GnRH cells, and evaluate the relevance of these findings to normal development. The possibility that E2 treatment induces opioid modulation of nMDA- stimulated LH release in these mice or alters the connectivity between host MBH and graft-derived GnRH cells will also be assessed . Immunocytochemical and ultrastructural studies will examine E2-induced alterations in GAP-43 expression in the vicinity of grafted GnRH cells and fibers, changes in the beta-endorphin input to GnRH neuronal elements and the relationship of tanycytes to GnRH terminals in the median eminence. These two interrelated lines of research should provide important new information not only regarding critical conditions for successful neuronal transplantation, but insights into how GnRH neurons target during development and are modulated by the hormonal milieu.